This invention relates to a device for controlling the fluid pressure in the output line of a high pressure pump utilized, for example, in hydraulic spraying of paint or other liquid coating materials.
In a typical operation, coating material is supplied at high pressure in the order of about 1,000 to 3,000 p.s.i. to an atomizing nozzle having an elliptical orifice forming a fan-shaped spray. Spray coating is conventionally an intermittent operation and, for this reason, a manually operated valve is associated with the nozzle to accommodate starting and stopping of the spray. Means must also be provided to control the fluid pressure at the spray tip because such pressure must be fairly constant even though the spray gun is operated intermittently. Thus, the fluid pressure at the outlet of the pump must be quite accurately controlled.
One prior art device for controlling output pressure comprises a spring loaded plunger which, responsive to pressure variations, opens and closes the contacts of a pump controlling switch. Such a device requires seals, such as O-rings, which get gummed up and wear out quickly. Furthermore, a dead end pocket may be created wherein the coating material can harden and eventually clog the system and disable the control.
Other prior art control devices, such as disclosed in U.S. Pat. Nos. 2,390,643; 2,823,543; 3,569,647 and 3,711,828, utilize a bourdon tube, the free end of which opens or closes a switch in response to variations in fluid pressures, the switch in turn controlling pump operation. Bourdon tube devices, however, are comparatively expensive and, at times, difficult to adjust. Furthermore, the bourdon tubes do not transmit fluid therethrough and therefore are not self-cleaning but, instead, are subject to inaccuracies in those instances when fluids such as paint are utilized in the system.
Still other devices, such as disclosed in U.S. Pat. Nos. 1,169,692 and 2,471,838, utilize systems of bellows to control fluid pressure, the bellows being closed at one end and therefore being subject to the same disadvantages as the bourdon tube devices when the fluid involved is paint or the like.